Add packageManager key to core/package.json now we recommend corepack enable

While I couldn't reproduce this locally (I am on an older NodeJS/corepack setup than Alex) I think this is a good idea:

• Yarn will apparently warn if packageManager is set but corepack is not enabled, which will help users get it installed locally.
• Once corepack is enabled it means everyone will use the specified version of yarn locally.

This broke my commit workflow and unfortunately took a bit of head-desking to figure out what was going on; I spent quite a bit of time tearing apart and rebuilding my toolchain before I understood the problem. From the yarn page about corepack:

DANGER
Some third-party distributors may not include Corepack by default, in particular if you install Node.js from your system package manager. If that happens, running npm install -g corepack before corepack enable should do the trick.

I think this needs a change record and probably a handbook page update somewhere. I already updated https://github.com/alexpott/d8githooks/blob/main/README.md but that's not what 99.99% of people using core's build tooling locally will reference. :)

In terms of change records we can probably just add to the main Yarn 4 one at https://www.drupal.org/node/3428571; doing that now.

Updated the CR; still needs to go in the handbook setup instructions somewhere though.

This was include in Frontend Developer tools for Drupal core in Sep 2022. I searched the Drupal wiki for other places this information needed to be included and could not find any other suitable page.

Therefor I am going to restore the fixed status. If someone has a better idea, re-open and add a comment.

Amending attribution, and also crediting @quietone for the docs info.